Introduction

In this chapter, you learn the mysticism behind the Windows API and how simple it is to use them in your application. Although VB can do just anything you need, some applications require features that VB cannot perform without tedious programming on your part. Fortunately, you can use routines that are already available elsewhere in Windows-such as those you write in C or C++ and store in DLLs. Almost all operating systems have API library functions that you can call so you don't have to keep re-creating the wheel. By utilizing these routines, you can extend the power of VB and make it perform some functions that only Windows has the true authority to perform.

The Windows API, or Application Programming Interface, is a set of internal routines you can call from Visual Basic.

All the Windows API routines are stored in special files called DLLs. These API appear in files stored in your Windows and Windows\System folders.

Dynamic Link Libraries, are code libraries that contain functions and procedures.

The word dynamic is a reference to the fact that, unlike traditional code libraries that are linked into your application during compile time, DLLs are loaded while the application is executing. When the application calls a function in DLL, the DLL is loaded into memory, the function executes, and returns information to the application, just like a standard compiled call.

The three most common DLLs:

•  USER32.DLL :  Contains functions that control the Windows environment and the user's interface, such as cursors, menus, and windows.
•  GDI32.DLL :  Contains functions that control output to the screen and other devices.
•  KERNEL32.DLL :  Contains functions that control the internal Windows hardware and software interface. Most of the memory, file, and directory service routines are located in KERNEL32.DLL.